Quantum (2): it gets weirder
Yet I Still Want to be a Catholic Post 8. Quantum (2) It Gets Weirder Still.
Ever since Bohr when faced with the paradox of how matter can be both particle and wave had proposed that real things do not exist until they are measured in some way, scientists have sought other explanations, although since these are even more bizarre than Bohr’s many still accept Bohr’s Copenhagen thesis. But there are other explanations, chiefly two. One is the many worlds interpretation first proposed by Hugh Everett III in 1957. According to Bohr, if there is no detector a photon passes through both slits as a wave. But if there is a detector or observer the detector collapses the wave function and the photon passes through only one slit as a particle. Everett proposed that even if there is a detector the photon passes through both slits as a wave, but enters our universe as a particle as if it had only passed through one slit and as it passes through the other slit enters a parallel universe where it might behave quite differently. There could be an infinity of these parallel universes. Alles klar? Well hardly.
Another explanation is that of David Bohm’s hidden variables. In Bohm’s view, there is an underlying and hidden dimension of reality that he calls the implicate order. Real things, even as small as electrons, behave as real things at all times in the way common sense assumes. But they are guided by a hidden pilot wave that only becomes manifest at the quantum level. It is the pilot wave that guides a photon through both slits as a wave. But it is also the pilot wave that detects the presence of a detector and instantaneously collapses the wave function. It is the pilot wave that does the collapsing, not the detector itself. Bohm thinks that the universe is filled with an infinity of pilot waves connecting all matter into an undivided whole.
Do these explanations really explain anything more than Bohr’s though? Who has ever seen an infinity of parallel universes or Bohm’s hidden implicate order? The plot had thickened still further when Heisenberg had proposed his uncertainly principle. Bohr had already worked out that at the quantum level there is what he called complementarity. Some aspects of reality, position/ momentum and time/energy for example, are intimately connected with each other and dependent on each other. But Heisenberg now proposed that at the quantum level there is radical uncertainty, the more precisely you measure momentum the less you can know about position, the more you know about the energy of an entity the less you know about the elapse of its time. The best you can do is make an estimate of probability. In his book Quantum for the Perplexed Jim al Khalili suggests an illuminating parallel. Suppose a criminal has recently been released from prison and the police know he is going to offend again but they don’t know where. But it is reasonable to think he is more likely to rob rich houses rather than poor ones so the probability he will offend in a rich district is greater. And now (not sure Jim goes this far, a brilliant extension of the analogy by me) suppose somebody else now phones in to say they’ve just seen him leaving a property he’s just robbed. The police now know almost exactly where he is. But he’s only just left the front door. They have no idea in which direction he will head next. The more exactly they know his position the less they know where he is heading. It could be Kensington or Mayfair or Eaton Square or Henley-on- Thames. But suppose somebody else phones in six hours later to say they’ve seen him getting on the tube at Marble Arch. They have a much less accurate idea of where he now is but they do know he’s more likely to be heading for Kensington or Knightsbridge rather than Henley-on-Thames. But as Jim points out, this analogy is only partial. An electron might not just be somewhere in the richer parts of London but its wave aspect means it could be anywhere in the whole of space. Where do the police start looking now? But all is not lost. To the rescue came the the other great equation of quantum physics, that of Edwin Schrödinger’s wave function. Since this is mathematical, unlike the districts than can be covered by the police’s panda cars, it applies everywhere. From the limited information you do have, by applying the wave function you can work out the probability of where the electron is far more accurately. But, mark, it’s still only a probability.
Between them Heisenberg and Schrödinger explained one of the deepest problems that had puzzled physicists ever since Planck. Under some circumstances electrons lose the energy which is necessary to keep them in their orbits. Why, then, as their energy drains away do they not collapse into the nucleus of the atom? At last this was explained. Because the nucleus of an atom is so small, as the electron dwindles into the nucleus its position could hardly be more precisely defined. But because of complementarity its momentum is correspondingly enormous, so great it immediately flies out of the nucleus, until, as the momentum lessens with increased distance, the attraction of the positive nucleus to the negative electron matches the momentum and the electron is kept in a stable orbit.
All this uncertainty was too much for Einstein. The whole point of science is to establish certainty. With two colleagues, he proposed what became known as the EPR thought experiment. Suppose, imagined Einstein, you fired two photons at exactly the same time at exactly the same speed from a point X in two precisely opposite directions, Y and Z. According to Heisenbergian uncertainty, if you were able to measure the position of one of them at a certain point, you would have no precise idea of its momentum, and vice versa, and so with the other. If you measured that one’s momentum you would not know its position. But since they were moving at the same speed, if you knew the position of one of them you would be able to calculate that of the other. So you could know precisely both momentum and position.
In the 1960’s an Irishman called John Bell produced an equation showing that if you could actually send two particles in opposite directions and change one of either their position or their momentum during its flight, if, because of complementarity, the other corresponded instantaneously then Heisenberg would be right and Einstein wrong. By the 1980’s it was possible to do the experiment and it was done by Roger Aspect in Paris over a distance of 11 metres. In fact, Aspect changed not the position or momentum but the spin of one of the particles (needless to say at the quantum level particles don’t spin in the way you or I would mean) and abracadabra! the other immediately corresponded, in accordance with the complementarity of quantum physics. It was Heisenberg who was right. Then a few years later the same experiment was done in Geneva over 11 kilometres and the same thing happened. The scientists concluded that even if two linked particles were separated by the whole universe, they would be in this kind of instantaneous communication. They call this phenomenon nonlocality. It would be hard to exaggerate the importance of this discovery. It bursts the bounds of all previous science that we have known, it abrogates the very cornerstone of Einsteinian physics that nothing can travel faster than the speed of light. Answers on a postcard please.
If you thought that quantum physics had already gone beyond all bounds of credibility, it was about to get weirder still. Richard Feynman noted that if you drop a teaspoon to the ground the electrons in the spoon travel straight to the ground, well of course they do they are in the spoon. But according to Feynman, applying Schrödinger to the two slit experiment, on the way they visit everywhere in the universe, popping randomly out of virtual reality into actual reality, and back again, in all sorts of, but in some places more likely, than others, Alpha Centauri perhaps, next door’s washing up basin, the Pope’s tiara, Orion’s rings. This explains why, as a particle, a particle passes through one slit but also in so far as it is a wave two, because it’s passing through everywhere else as well. The electrons in the spoon go straight to the ground in a straight line because since all the other directions in which it is going cancel each other out, the straight line is the only direction which doesn’t have a cancelling partner. Complementarity is everywhere in quantum physics.
In 1978 and then again in 1984 John Archibald Wheeler conducted his delayed choice experiments. Wheeler fired photons through the two slit screen with two detectors trained on the two slits. In accordance with the accepted doctrine of quantum physics, when the detectors were turned off the photons went through as waves, and when they were turned on the wave function was collapsed and they went through only one slit one at a time as particles. But then Wheeler left the detectors off and switched them on during the flight of the particles between the two screens. Since the particles had gone through undetected as waves they presumably would arrive at the back screen as waves. But they didn’t. They arrived as particles. Somehow it was as if they knew what Wheeler was going to do before he did it. He called it backward causation.
What conclusions, if any, can we draw from all this? How unlikely and remote from everyday life it sounds. Yet since quantum mechanics was discovered and put to work it we encounter its activities every day. Consider the flow of electrons in the electric wiring in your house. It works because of quantum tunnelling. There is a thin layer of aluminium oxide between the connecting wires, which according to classical physics should stop the flow of current. But according to Heisenberg’s uncertainty principle time and energy are inversely related. The electrons are in a superposition because they are waves as well as particles, and therefore are in one of their states able to be in two places at once. Because the aluminium oxide is so thin and the time it takes to pass through it so minute, the electrons have enough corresponding energy to pass through it instantaneously, so they are neither in one place nor the other and there is a flow of current. Computers, mobile phones, lasers, magnetic resonance imaging used in medicine, positron emission topography used in brain scans, anything that employs semi-conductors. Quantum mechanics lies only just under the surface of our ordinary lives, in fact everything, because everything is composed of atoms and atoms are quantum structures.
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